1. Field of the Invention
The invention concerns optical amplifiers and more particularly semiconductor optical amplifiers.
2. Description of the Prior Art
Optical amplifiers are routinely used for the transmission of optical signals and for producing "all optical" switching devices.
The essential characteristic of an optical amplifier is its gain, which is defined as the ratio of the optical power that it provides at the output to the optical power that it receives at the input. For low optical powers the gain is constant and the amplifier operates in a linear manner. On the other hand, from a particular level of applied power the value of the gain decreases as the power increases. Accordingly, another characteristic of an amplifier is its output threshold power, which is defined as the output power for which the gain has a value equal to half the gain of the amplifier when the latter is not saturated (unsaturated gain).
Saturation is particularly significant in the case of semiconductor optical amplifiers. The saturation power of the amplifier must be as high as possible for applications requiring linear operation and high power.
One solution to this problem is to optimize the dimensions of the component and the composition of the semiconductor layer that constitutes it or to adopt complex active structures such as multiple quantum wells. For example, widening the guide part of the active layer reduces the power density so the threshold can be raised. However, this solution is limited by the fact that the guide of the amplifier is usually required to have a monomode structure.
Another solution is to use a stabilized gain amplifier as described in European patent application EP-A-639876 published Feb. 22, 1995, for example. This solution extends the range of linear operation of the amplifier but in a limited fashion and to the detriment of the useable gain.
The aim of the invention is to avoid the limitations of the methods referred to hereinabove. To this end the present invention proposes a solution that raises the saturation threshold very strongly without degrading the gain whilst remaining compatible with any semiconductor amplifier structure or technology.